add .~ notation

[cscherrer]

This one-line update (obvious in retrospect) lets us do things like

julia> m = @model begin
       x .~ Normal.(rand(4))
       end;

julia> m
@model begin
        x ~ For(identity, Normal.(rand(4)))
    end


julia> rand(m())
(x = [0.3757908068715139, 1.7752927895608144, 0.13911698109223786, -0.1342542512198157],)

as discussed (two years ago!) in #14

@DilumAluthge I think this will also be useful for our MLJBase.UnivariateFinite manipulations